Ontario was a federal electoral district represented in the House of Commons of Canada from 1925 to 1997.

It initially consisted of the townships of Pickering, Whitby (East and West), Reach, and Scugog, and the city of Oshawa in the county of Ontario.

In 1966, it was redefined to consist of, in the County of Ontario, the Townships of Pickering, Reach, Scott, Scugog, Uxbridge, East Whitby and Whitby (excluding the area between the west limit of the City of Oshawa and the east limit of the Town of Whitby lying south of the road allowance between Concessions 2 and 3), and, in the County of York, the Townships of Georgina and North Gwillimbury, and all the islands of Georgina Island Indian Reserve No.

In 1976, it was redefined to consist of the Township of Uxbridge, and the Towns of Ajax, Pickering and Whitby.

The electoral district was abolished in 1996 when it was redistributed between Pickering—Ajax—Uxbridge and Whitby—Ajax ridings.